Investigation of electronic couplings with tunable, background-free, extreme-ultraviolet four-wave mixing

Time-resolved four-wave mixing (FWM) spectroscopy is a versatile pump-probe technique which has been extended to the extreme-ultraviolet (XUV) regime in recent years. It is based on χ⁽³⁾ parametric processes, in which the interaction of weak XUV attosecond pulse trains (APT) and strong-field near-infrared (NIR) femtosecond pulses with atoms or molecules captures information on the light-induced couplings between different quantum states. We start with helium as a prototypical atomic system where numerical modeling is feasible, and then generalize to study polyelectronic systems, such as krypton. Background-free XUV FWM signals are generated and easily isolated from the driving XUV spectrum by using non-commensurate NIR pulses. Moreover, the frequency tunability of the NIR pulse allows us to resonantly drive and selectively control the detuning from intermediate states, which gives us control over light-induced structures, Autler-Townes splitting, and coherent XUV FWM emissions. We also extend these studies to high-density regimes where collective effects play an important role.